U.S. Pat. No. 3,142,015 shows one type of high voltage alternating current capacitive measuring apparatus. The current comparator device in this U.S. patent consists of a primary, a secondary, a deviation and a detection winding wound on a core with a standard capacitor being connected to a tap on the secondary winding, the other end of the standard capacitance being connected to an alternating voltage source. An unknown capacitor is connected between the voltage source and the primary winding with a null detector being connected across the detection winding. The apparatus also includes circuit means to obtain a balanced condition for a quadrature component so that a dissipation factor for the unknown capacitance can be determined. This circuit includes a further capacitor connected to the other side of the secondary winding so that almost all the voltage drop occurs across the standard capacitor while a low voltage supply is obtained across the further capacitor which is accurately proportional to the high voltage supply. This source of accurately controlled low voltage makes it practicable to obtain a correction current for the quadrature adjustment such as by applying that voltage, or a voltage of opposite polarity, through a variable resistor to the tap on the secondary winding.
U.S. Pat. No. 3,579,101 shows another similar type of measuring apparatus wherein a variable resistor is in series with the standard capacitor and secondary winding and the low voltage is applied, via a polarity reversing switch, to an autotransformer whose output is applied across the variable resistor.
A paper entitled "A Transformer-Ratio-Arm Bridge for Measuring Large Capacitors Above 100 Volts" by Oskars Pertersons in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-87, No.5, May 1968 (pps 1354-1361) describes the basic operating principle of these types of high-voltage capacitance bridges. This paper also discusses various methods of compensating for errors caused by lead impedances and other factors which will be discussed in more detail later.
Another paper entitled "A Wide-Range High-Voltage Capacitance Bridge With One PPM Accuracy" by Oskars Petersons et al, in IEEE Transaction On Instrumentation And Measurement, Vol. IM-24, No.4, December 1975 (pps 336-344) describes in more detail the basic operating principle of these types of high-voltage capacitance bridges along with the nature of bridge errors and their reduction. This paper also describes circuitry to balance the bridge to obtain the capacitance value (C.sub.x) of the unknown capacitance as well as it conductance (G.sub.x) from which the dissipation factor DF of the unknown capacitor can be obtained. A discussion of range extending transformers is included in that paper.